1. Field
The present invention relates generally to mobile phone technology, and more specifically to detecting the modulation format type of a signal.
2. Background
Currently a popular standard for digital mobile phone technology is Global System for Mobile Communications (GSM) which is a second generation (2G) mobile phone system. An add-on enhancement to the GSM communications system that is gaining widespread acceptance is Enhanced Data Rates for Global Evolution (EDGE). EDGE technology increases data throughput to 384 Kbps and meets the International Telecommunications Union's standards for a third generation (3G) network. Services associated with 3G telephone technology include the ability to transfer both voice data (a telephone call) and non-voice data (e.g., email, instant messaging, etc.).
EDGE technology transmits and receives signals in both Gaussian Minimum Shift Keying (GMSK) and 8 Phase Shift Keying (8PSK) modulation formats. As is well known in the art, GMSK is a digital modulation technology using Phase Shift Keying where an information signal can contain two possible distinct phase shifts. As is well known in the art, 8PSK is also a digital modulation technology using Phase Shift Keying where the information signal contains eight possible distinct phase shifts. GMSK technology produces a one bit word/symbol for every change in phase while 8PSK produces a three bit word/symbol for every change in phase (as compared to the phase of a reference wave).
In a cellular mobile phone system using EDGE technology, EDGE compatible hardware and software are included at base stations (e.g., in tranceiver units) and mobile terminals (e.g., cellular phones) to modulate and demodulate signals using the EDGE shift keying schemes. Signals are typically modulated and transmitted as data blocks (the basic unit for transmitting data) where a data block is comprised of four bursts of data. All four bursts of a data block are typically modulated in the same format type (either GMSK or 8PSK) so that each data block of a signal has one uniform modulation format type.
Since EDGE compatible base stations transmit both GMSK and 8PSK signals, a mobile terminal must be able to detect the modulation format (GMSK or 8PSK) of a received signal in order to demodulate the signal correctly. Conventionally, modulation format type detection is performed by independently examining each burst of a data block of the received signal and determining the modulation type for that burst. After the modulation type is determined for a burst, the burst is treated as a burst of the determined modulation format and processed (i.e., demodulated) as such. The next determination is then made independently on the next burst of the data block and processed accordingly. As such, conventionally the modulation type of a burst is determined independently/separately from the other bursts of the same data block and then processed independently based on the determination.
Conventional methods of determining the modulation format type of bursts of a received signal, however, cause a significant amount of misdetections (i.e., incorrect determinations of the modulation format type) especially at low signal strength levels (i.e., in cases where the received signal has a low signal-to-noise ratio). As such, there is a need for a more accurate method for detecting the modulation format type of a received signal, especially at low signal strength levels when the probability of misdetection is much higher.